1. Field of the Invention
This invention relates to high density formation record media of an electrostatic capacitance type, such as video or digital audio discs, on which an information signal is recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems of the electrostatic capacitance type, an information signal is recorded as geometric variations by forming pits in a spiral plane or groove or in concentric planes or grooves, thereby obtaining high density information records such as video or digital audio discs. The individual pits of the record are very small and are arranged in a very high density. When the geometric variations are traced with a pickup stylus such as a diamond having an electrode, capacitive variations are established between the stylus and the record according to the geometric variations, so that the recorded information signal can be played back or reproduced.
Several kinds of information signal records of the electrostatic capacitance type have been proposed for use in the above type of electrostatic capacitance playback system. In one such record medium, a metal electrode is provided on one or both surfaces of a record substrate on which geometric variations have been press-molded according to an information signal. This permits an electrostatic capacitance to be established between the record electrode and an electrode of a pickup stylus according to the information signal. More particularly, the record substrate, on which pits are formed as desired, is covered with a metallic thin film having several hundred angstroms in thickness, and also with a several hundred angstrom thick dielectric layer overlaying the metallic film. This dielectric layer serves to prevent short circuiting of the electrodes and to increase the dielectric constant between the electrodes. However, the record of this type needs a number of fabricating steps including a press molding of the record substrate, steps of depositing the metallic thin film and the dielectric layer, and the like. Thus, the fabrication is complicated and troublesome with the need of a relatively large-scale manufacturing apparatus. Thus, the production cost becomes very high.
Another type of information record medium is known. This medium is obtained using conductive resin compositions which comprise polyvinyl chloride resins, lubricants and several tens percent of carbon black. The conductive composition is press-molded so that an information signal is recorded as geometric variations. In this type of recording medium, an electrostatic capacitance is established between the electrode of a pickup stylus and the record itself. Thus, the step of depositing a metallic thin film on the substrate is not necessary. In addition, because fine particles of carbon black are individually covered with the resin, the dielectric film is not necessary as well. Thus, this record medium can be simply manufactured at a relatively low cost.
However, the record medium has the serious problem that when it is set in a recording and reproducing apparatus and reproduced over a long time, the medium surface is worn by means of a playback stylus which is in contact with the surface. As a consequence, the pits may deform, causing the electrostatic capacitance to be varied. Thus, an accurate reproduction cannot be expected. Especially, when a picture frame is frozen, the playback stylus is brought to contact with a certain position under a pressure of 400 to 500 kg/cm.sup.2 at a rate of 15 cycles/second, so that the medium surface is worn to a substantial extent.
In order to solve the above problem, there has been proposed a recording medium in which not only thermal stabilizers and processing aids, but also lubricants are added during mixing of thermoplastic resins and carbon black and the resulting conductive composition is used to make the medium. This medium is intended to improve the wear resistance by addition of lubricants to the conductive resin composition. However, the mere addition of lubricants during the mixing is not effective because carbon black having a large surface area is contained in large amounts, so that the stabilizers and processing aids have to be used in large amounts. In addition, in order to improve the wear resistance, large amounts of lubricants are also required. The total amount of these additives exceeds an amount which enables a vinyl chloride resin to contain such additives therein. This means that when the composition is press-molded, the various additives are plated out and deposit on the metal mold. This will considerably lower the productivity of the medium.